1. Field of the Invention
This invention relates to the digestion and disposal of spent ion exchange resins and the recovery of actinide, rare earth metal, and/or heavy metal species from these resins. More particularly, the invention relates to the digestion of actinide, rare earth metal, or heavy metal loaded ion exchange resins in such a way that these species can be subsequently analyzed without interference from the resin digestion products, thereby allowing an accurate analytical assessment of the identity and amount of these materials loaded in the resins.
2. Description of the Related Art
Ion exchange resins are widely used to clean up waste and cooling water from the nuclear power and nuclear weapons industries, and in particular to remove radioactive, rare earth metal, and/or heavy metal species from the water so that the water can be reused or disposed of as a lower level waste product. However, these ion exchange resins eventually become fully loaded with the metal species, and cease to perform their function effectively. When this happens, the resin must be replaced, and the spent, loaded resin presents another disposal problem. Various methods exist for disposing of the spent resin, including digestion of the organic constituents to decrease the resin volume and/or immobilization of the remaining radioactive or hazardous species in some final form, such as ceramic or glass, that is stable, low in volume, and relatively easy and safe for handling and disposal.
Ion exchange resins suitable for waste water treatment may be of a number of different types, including those having diphosphonic acid and/or sulfonic acid groups on a polymer backbone. In particular, resins having polystyrene-containing backbones that have been crosslinked with divinylbenzene and substituted with diphosphonic and/or sulfonic acid substituent groups have been found to be particularly suitable in this regard. Resins having diphosphonic acid substituent groups have been found to be particularly suitable because of the efficiency and tenacity with which the these moieties bind cations, including cations of radioactive species, namely actinide cations, as well as rare earth metal cations, and heavy metal cations. DIPHONIX resin (diphosphonic and/or sulfonic acid substituted polystyrene/divinylbenzene resin, Eichrom Industries, Inc.) is particularly effective in removing actinides from wastewater, and from large soil samples. Suitable diphosphonic acid containing resins are disclosed in U.S. Pat. Nos. 5,281,631 and 5,449,462, the entire contents of which are hereby incorporated by reference.
In many cases, it is desirable to analyze the amount and identity of actinides, rare earths, or heavy metal loaded on the resin in order to obtain information useful to assess and control the upstream nuclear processes. It is also often desirable to analytically and quantitatively assess the level of radioactive species, such as actinides, or hazardous metal species, such as rare earth or heavy metals, in samples of water, bodily fluids, soil, fecal samples, etc., in order to assess the level of exposure of the environment or individuals to these potentially harmful species, as well as to determine a suitable disposal method for these materials.
While it would be desirable to do this using the diphosphonic acid substituted ion exchange resins discussed above (because of their effectiveness and efficiency in capturing these species), this is made very difficult by the very tenacity of the resins for actinides, rare earth metal, and heavy metal cations that make them so effective in the first place. In short, it is very difficult to elute the actinide, rare earth metal, or heavy metal cations from the loaded resin in any significant amounts. Destruction of these removal resins by digestion of the organics is also difficult, because if the resin is not completely destroyed, then organic residual groups containing phosphonic acid moieties can rebind to the metal species. When this occurs, the phosphonic acid/metal bound complex can then become bound to the chromatographic or analytical resins used in the downstream analytical processes. This bonding is sufficiently strong that it can cause substantial difficulties in eluting the actinides from the downstream analytical resins for further analysis. Diphosphonic acid-substituted resins, such as DIPHONIX, are particularly troublesome in this regard.
U.S. Pat. No. 5,523,514 discloses low temperature digestion of plutonium-contaminated ion exchange resins, such as DOWEX, with sulfuric acid in an open microwave vessel. However, nitric acid was not found to work with these resins in this process, as indicated by the Table disclosed in the '514 patent. Diphosphonic acid-substituted resins are even more difficult to digest than DOWEX, and cannot be effectively digested using low temperature nitric acid processes. Further, sulfuric acid is unsuitable for digestion in a closed microwave reaction vessel because the sulfuric acid will attack the vessel lining at the elevated temperatures and pressures achieved in a closed vessel. In addition, digestion of the actinide-loaded removal resin with some strong acids, in particular sulfuric acid, is detrimental to the downstream analytical separation processes, in particular downstream separations using column chromatography.
As a result, there exists a need for a method of processing removal resins loaded with actinides, rare earth metals, and/or heavy metals, and in particular a method of processing diphosphonic acid substituted resins loaded with one or more of these materials, in a way that allows the use of downstream analytical resins without undue difficulty in eluting the actinides or rare earth metals, and without damage to downstream analytical resins. It is one object of the present invention to provide such a process.
More specifically, it is an object of the invention to provide a process capable of fully digesting a diphosphonic acid-substituted ion exchange resin.
It is also an object of the present invention to release actinides, rare earth metal, and/or heavy metal cations from a diphosphonic acid-substituted ion exchange resin in a manner that allows accurate analysis of the type and quantity of these cations by subsequent chromatographic processes.
It is also an object of the present invention to provide a digestion step for diphosphonic acid substituted cation exchange resins that will not damage chromatographic resins used in downstream analytical processes.